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650W test load

Discussion in 'General Electronics Discussion' started by eem2am, Jul 1, 2013.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    I must make a resistive SMPS test load which is to be 650W maximum.

    The Load will have a 50KHz sinusoidal current in it which is 1.9 Amps RMS.
    This is a constant , regulated current.

    The voltage will be 342V rms.

    The load needs to be comprised of about 13 individual resistors. We need the ability to be able to short out each of these resistors so as to be able to vary the load.
    We are thinking of using relays to short out each resistor.

    I am told that the load must “stand up” and must have fans.

    The resistors should not be inductive, and indeed in the load, we wish the voltage and current to be in phase.
    I have been advised to use planar resistors.

    http://uk.farnell.com/h-s-marston/cf2-0816-0805-1500na/heat-sink-0-13-c-w/dp/936420

    I am thinking of using something like the above to fasten the planar resistors to. Do you know the best way to fasten the resistors to the heatsink, and other details?
     
  2. john monks

    john monks

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    Mar 9, 2012
    Have you considered nichrome wire? If you use carbon resistors near the upper end of the power rating they will drift substantially with time. Film resistors tend to open with a short over voltage. Nichrome can be set up in a frame work like an electric heater with a fan to dissipate heat and is virtually immune to short over voltages.
    What is it that you are trying test?
     
  3. eem2am

    eem2am

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    Aug 3, 2009
    trying to test a 650W , 1.91A rms sinusoidal current source switch mode power supply
     
  4. john monks

    john monks

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    Mar 9, 2012
    That tends to confirm my thinking about nichrome wire. My experience with planar resistors is terrible to I don't use them. They are OK if you can be sure not to overstress them. But I tried some destructive testing with planar resistors and they all turned into open circuits with short over voltage conditions.
     
  5. GonzoEngineer

    GonzoEngineer

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    Dec 2, 2011
    I do testing of systems to 60kW. All we use are Nichrome Wire Loads and big ass fans.

    I have a shitload of Planar Resistors and water cooled plates for them if you want them, but they don't last very long.
     
  6. eem2am

    eem2am

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    Aug 3, 2009
    starnge you should say, we too have had failures with planars before...but i cant see why, i will just use bigger ones, and with the fan it should be ok.
     
  7. john monks

    john monks

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    Mar 9, 2012
    I tried placing a 0.22 microfarad capacitor charged to 800 volts across 20 carbon composition resistors in parallel all 1 ohm each and every resistor turned into an open circuit. Then I tried a similar test with a commercially made 50 ohm dummy load made for RF transmitters with a 900 volt pulse with only a few microseconds in width and that turned into an open circuit. And that used a planar resistor. So I acquest to using nichrome wire.
    You can use planar resistors but keep in mind that your peak voltage never exceeds the maximum rating of the resistor and if you are running near the maximum rating the resistance will change with time.
    To me the big drawback to using nichrome wire is getting rid of the inductive loop of all that wire and that can be very tricky if not downright impossible.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    It is possible to wind the wire in such a way as to minimise inductance, but you're not going to eliminate it completely.

    See here for a way to minimise it
     
  9. john monks

    john monks

    693
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    Mar 9, 2012
    My curiosity got the better of me so I just place a Dale RS2B-125 125ohm wirewound resistor across a 0.22uF capacitor charged to 1034 volts 6 times. The resistor went from 125.04ohms to 124.87ohms. I don't know what metal the wire is but obviously something made it decrease and I don't believe it is temperature effects but still much better than metal film and carbon composition resistors.
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Since the source freq is constant at 50KHz it might be feasible to tune out the reactance.

    Chris
     
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